RARE-EARTH ELEMENT SPECIATION IN GEOTHERMAL FLUIDS FROM YELLOWSTONE-NATIONAL-PARK, WYOMING, USA

Elevated concentrations (20-1133 nmol/kg) of rare earth elements (REE) are present in acid-sulphate and acid-sulphate-chloride hydrothermal waters of the Yellowstone National Park (YNP). We used recently estima ted thermodynamic data (Haas et al., 1995) to speciate seventeen YNP h ydrothermal fluids with the EQ3NR code. The fluids show a range in pH (2.0-4.0) and temperature (70 degrees-93 degrees C) and are of varied chemistry, with TDS = 155-2,075 ppm, sulphate = 100-10,325 mu mol/kg, chloride = 190-24,580 mu mol/kg, fluoride = 26-1,790 mu mol/kg, and SO 4/F = 0.8-323. Field temperature and pH measurements were used in the modelling and saturation with kaolinite and quartz was assumed, althou gh quartz was actually supersaturated. Where possible, oxygen fugacity was calculated from the analytical sulphate/sulphide ratios, otherwis e it was set above the hematite magnetite buffer and pyrite saturation (although speciation calculations show that this is not critical). Ca rbonate and phosphate levels were set at the analytical detection limi t, with the exception of 4 waters for which analytical data for phosph ate existed. The waters show Little fractionation of REE relative to t heir host rhyolitic volcanics; it appears that the REE abundances of h ydrothermal fluids resulting from alteration of YNP rhyolites are unaf fected by the presence of potential complexing species, i.e., that aci d-alteration completely strips REE from the portion of the rocks that it affects without any fractionation across the REE series. The main c ontrol over REE speciation is the relative abundances of potential com plexing agents; however, pH and absolute abundances are also import-an t. In the most acidic waters (pH similar to 2.0) the free ion is the m ajor species when salinity and SO4/Cl are low (60-80% of each REE), an d REE complexes with chloride can be significant (up to 5%). For highe r SO4/Cl values, sulphate complexes dominate (80-90%). For less acid w aters (pH 2.8-4.0) fluoride is the main complexing agent in low SO4/F fluids, but as the SO4/F ratio increases the sulphate species become d ominant, especially for the Light REE (LREE). Copyright (C) 1998 Elsev ier Science Ltd.